Imaging apparatus and method for controlling same

ABSTRACT

In an imaging apparatus including a stepping motor that drives a diaphragm, a favorable moving image exposure control is carried out while reducing electrical power required for holding the diaphragm position. For moving image exposure control, a determination unit determines the presence or absence of the diaphragm driving. When a rotor and the magnetic poles of the stator do not face to each other in the first operation mode, the control unit of a stepping motor drives the rotor to a facing position, and shuts off the current in a coil. When the diaphragm driving does not occur in the second operation mode, the control unit of a stepping motor shuts off the current in the coil at the state where the rotor of the motor and the magnetic poles of the stator face to each other. A holding current to be supplied to the motor is shut off in the first and second operation modes, whereby the lower power consumption can be realized. The program profile of a diaphragm drive method and an exposure control is switched depending on the type of an imaging lens unit, whereby a favorable moving image exposure control can be realized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drive and position control of adiaphragm. In particular, the present invention relates to a techniquein which power consumption can be lowered in consideration ofresponsiveness of moving image exposure.

Description of the Related Art

Currently, in what is referred to as a “single-lens reflex type” digitalcamera of which the interchangeable lens is removable, a product thathas a live view function that displays a preview image prior tophotographing a still picture on a TFT monitor disposed on the rear faceof a casing or is capable of photographing a moving picture is known.Conventionally, there has been required high-speed responsiveness andhigh positional accuracy for focus driving, diaphragm driving, and thelike of an interchangeable lens, since the primary aim was still picturephotographing. For this reason, a stepping motor excellent in drivingcharacteristics and controllability tends to be employed as an actuatorfor driving the diaphragm. The drive methods of a stepping motorincludes 1-phase excitation drive, 2-phase excitation drive, 1-2-phaseexcitation drive, and the like, which may be provided for separate usein accordance with applications.

In order to ensure that a stepping motor is employed as the actuator forthe diaphragm to stop the diaphragm to hold it at that particularposition, it is required that a holding current continuously flowsthrough a coil of the excitation phase at the stop position. A holdingcurrent itself may be smaller than the current used during driving.However, this is a direct current, and, if this state persists for along time particularly when a battery is employed as the power supply,power may be consumed and heat generated despite the fact that thediaphragm is not being driven. This particularly creates problems when abattery is employed for a power supply.

In the 1-phase excitation drive, even if a current supplied to astepping motor is shut off in a stop state, the magnetic poles of therotor and the stator face to each other, and the stop position is heldby the magnetic force of the rotor. However, the rotation stepping angleper step is large, which is not suitable when highly accuratepositioning, such as a diaphragm, is required.

In the 2-phase excitation drive, the rotor stops at a positionintermediate between two phases. Hence, in order to stably hold the stopposition, a minimum holding current must continuously flow through thecoil in the stop state. As a result, power consumption and heatingoccur.

In the 1-2-phase excitation drive that combines the 1-phase excitationdrive and the 2-phase excitation drive, the rotation stepping angle perstep becomes as half that of the driving methods, and thereby a finecontrol can be carried out. In the 1-phase stop position, the stopposition can be held even if a current is shut off, whereas in the2-phase stop position, a holding current is required. Even if theholding current is decreased to a limit value at which the stop positionof the rotor can be held, the holding current cannot be ignored when theuse of the battery power supply is considered.

As described above, realizing a satisfactory drive control by using anyof the drive methods is difficult. In particular, when exposure controlis performed by a diaphragm during moving picture photographing or thelike, accurate controllability of the diaphragm and holding it at thestop position are required.

In order to solve the aforementioned problems, a method for driving afocus lens, which is disclosed in Japanese Patent Laid-Open No. 7-77648,is known. In this method, even when the stop position needs not to bestrictly controlled by adjusting the depth of field or the like and thestepping motor is at a 2-phase stop position, the stepping motor isrotated by 1-step within the range of the depth to thereby stop at a1-phase position. When the subject is within the depth of field, therotor is rotated by 1-step to a position of a 1-phase excitation phaseand then the electricity is shut off, whereby a holding current becomeszero. When the subject is outside the depth of field, the currentlimitation is performed.

However, when the method disclosed in Japanese Patent Laid-Open No.7-77648 is applied to a method for controlling a diaphragm under theexposure control of a moving image, there are problems withcontrollability and power consumption. For example, when an imaging lensunit that provides poor controllability of the diaphragm is mounted on acamera body, a problem such as flickering or the like occurs by theoperation by which the rotor is moved to a position of a 1-phaseexcitation phase. Also, when long term photographing such as movingpicture photographing or the like is expected, power consumption becomeslarge even if current limitation is performed. Therefore it is desiredthat the diaphragm be held when the holding current is zero

SUMMARY OF THE INVENTION

In view of the foregoing, according to an aspect of the presentinvention, an imaging apparatus is provided that includes a lens sectionhaving a lens group and a diaphragm; a stepping motor that drives thediaphragm by means of the excitation of a plurality of coils; a controlunit configured to control the stepping motor; an exposure control unitconfigured to control exposure during video photography by changing adiaphragm value, a charge accumulation time, or a gain of an imagingelement; and a determination unit configured to determine whether or notthe amount of change in position of the diaphragm is equal to or greaterthan a predetermined amount, when a current supplied to the plurality ofcoils is shut off.

The control unit switches operation modes in association with thedriving of the stepping motor depending on the type of the lens section,in a first operation mode using a phase by which the position of a rotoris not held when a current supplied to the plurality of coils is shutoff; the control unit controls the stop position of the diaphragm basedon the output of the exposure control unit and the determination resultobtained by the determination unit; and in a second operation mode usingonly a phase by which the position of a rotor is held when a currentsupplied to the plurality of coils is shut off, the control unitperforms diaphragm control based on the output of the exposure controlunit.

According to the present invention, in an imaging apparatus thatperforms diaphragm driving using the stepping motor, the operation modesare switched depending on the type of a lens section, whereby afavorable moving image exposure control is carried out while reducingelectrical power required for holding the diaphragm position.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating exemplary structure of an imagingapparatus according to an embodiment of the present invention inconjunction with FIGS. 2 to 10.

FIG. 2 is a diagram illustrating a diaphragm driving control circuit.

FIG. 3 is a diagram illustrating an example of the drive mode of thediaphragm control circuit.

FIG. 4 is an explanatory diagram of the micro-step drive of thediaphragm control circuit.

FIG. 5A is an explanatory diagram of the operation mode of the diaphragmcontrol circuit.

FIG. 5B is an explanatory diagram of the operation mode of the diaphragmcontrol circuit.

FIG. 5C is an explanatory diagram of the operation mode of the diaphragmcontrol circuit.

FIG. 5D is an explanatory diagram of the operation mode of the diaphragmcontrol circuit.

FIG. 6A is an explanatory diagram of each of the driving methods.

FIG. 6B is an explanatory diagram of each of the driving methods.

FIG. 6C is an explanatory diagram of each of the driving methods.

FIG. 7 is a flowchart illustrating an exemplary processing performedduring video photography.

FIG. 8 is a flowchart illustrating an example of the flow of exposurecontrol.

FIG. 9 is an explanatory diagram illustrating the exposure-responserelationship during video photography.

FIG. 10A is a program profile illustrating an example of exposurecontrol performed during video photography.

FIG. 10B is a program profile illustrating an example of exposurecontrol performed during video photography.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will now bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram illustrating an exemplary configuration of asingle-lens reflex type digital camera as an imaging apparatus accordingto an embodiment. An imaging lens unit 200 is detachably mounted on acamera body 100 via a mounting mechanism (not shown). A mounting partincludes a group of electric contact points 210. The group of contactpoints 210 transmits and receives a control signal, a state signal, adata signal, and the like between the camera body 100 and the imaginglens unit 200. The group of contact points 210 also supplies varioustypes of voltages including a power supply voltage transmitted from theimaging lens unit 200 to a system controller 120. Owing to the group ofcontact points 210, the camera body 100 can communicate with the imaginglens unit 200 to drive an imaging lens 201 (while only one lens is shownin FIG. 1, a plurality of lens groups may also be included) and adiaphragm 202 in the imaging lens unit 200. The group of contact points210 may transmit a signal through optical communication, audiocommunication and the like, as well as through electrical communication.The group of contact points 210 and the system controller 120 constitutea lens detection unit.

An imaging light flux from the object image reaches a quick returnmirror 102, which is pivotable in the direction of the arrow shown inFIG. 1, via the lens section that includes the imaging lens 201 and thediaphragm 202. A center portion of the quick return mirror 102 is a halfmirror, through which a part of the light flux is transmitted when thequick return mirror 102 is driven downward to the position shown inFIG. 1. The transmitted light flux is downwardly reflected by a submirror 103 disposed on the quick return mirror 102 to thereby reach anauto focus (AF) sensor unit 104. The AF sensor unit 104 is of a phasedifference detecting type which includes a field lens disposed near animage forming surface (not shown), a reflection mirror, a second imageforming lens, the diaphragm, a line sensor having a plurality of chargecoupled devices (CCD) and the like. A focusing point detection circuit105 controls the AF sensor unit 104 based on the control signal from thesystem controller 120, and detects a focusing point by the known phasedifference detecting method. It should be noted that the AF sensor unit104 and the focusing point detection circuit 105 constitute a focusingpoint detection unit.

At the same time, the imaging light flux reflected on the quick returnmirror 102 reaches a user's or photographer's eye via a pentaprism 101and an eyepiece lens 106. A light-metering sensor (not shown) isprovided near the eyepiece lens 106. The sensor output, i.e., the resultof the measurement of the brightness of the object image is transmittedto the system controller 120 via a light-metering circuit 107. Thelight-metering sensor, the light-metering circuit 107, and the systemcontroller 120 constitute a light-metering unit.

When the quick return mirror 102 is driven upward and retracted from theimaging optical axis, the light flux from the imaging lens 201 reachesan imaging element 112 using an image sensor such as a complementarymetal-oxide semiconductor (CMOS) and the like via a focal plane shutter108 and a filter 109.

The focal plane shutter 108 has a front screen and a rear screen so asto perform control to transmit and block the light flux emitted from theimaging lens 201. The filter 109 has two functions. One is cutting offinfra-red rays to lead only visible light to the image sensor 112. Theother is serving as an optical low pass filter. Note that the sub-mirror103 is adapted to be folded when the quick return mirror 102 is drivenupward.

The camera body 100 includes the system controller 120 constituted by anarithmetic processing unit such as a CPU or the like that controls theentire camera body 100 and appropriately controls an operation of eachpart of the camera body 100. The system controller 120 transmits thecontrol command to an optical system control section 207 of the imaginglens unit 200, controls a lens driving mechanism 203 via a lens controlcircuit 204, and controls a diaphragm driving mechanism 205 via adiaphragm control circuit 206. The optical system control section 207 isconstituted by a CPU or the like. The optical system control section 207and the diaphragm control circuit 206 constitute a control unitconfigured to control a stepping motor for driving the diaphragm.

The lens driving mechanism 203 moves a focus lens (not shown) in anoptical axis direction to perform focus adjustment. Also, the diaphragmdriving mechanism 205 drives the diaphragm 202 using the stepping motoras a driving source. The system controller 120 controls a shuttercharge-mirror driving mechanism 110 to perform control to drive thequick return mirror 102 to thereby control shutter-charge of the focalplane shutter 108. The system controller 120 is connected with a shuttercontrol circuit 111 configured to control the travel of the front andrear screens of the focal plane shutter 108. The driving source of eachof the front and rear screens of the focal plane shutter 108 isconstituted by a spring, and thus a spring charge is required for thenext operation after the shutter travel.

The system controller 120 is connected with an electrically erasable andprogrammable read only memory (EEPROM) 122 that stores a parameter thatmust be adjusted for controlling the camera and camera identificationdata (ID) information for identifying the camera itself. The EEPROM 122also stores AF compensation data, an automatic exposure compensationvalue, and the like adjusted by the reference lens.

The optical system control section 207 includes a lens informationstorage device (not shown) that stores information inherent to a lenssuch as a focusing point distance, full-opened diaphragm, lens IDinformation assigned to each lens, and information received from thesystem controller 120. The system controller 120 controls the lensdriving mechanism 203 via the optical system control section 207 to forman image of the object on the imaging element 112. Also, the systemcontroller 120 controls the diaphragm driving mechanism 205 based on anAv value corresponding to a set diaphragm value and further outputs acontrol signal to the shutter control circuit 111 based on a Tv valuecorresponding to a set shutter-speed value to thereby perform exposurecontrol.

The system controller 120 is connected with an image data controller115. The image data controller 115 includes a digital signal processor(DSP). The image data controller 115 controls the image sensor 112, andcorrects and processes an image data that is input by the image sensor112. Note that the system controller 120 and the image data controller115 have a light-metering function. The image data controller 115divides the image signal into regions and supplies them to the systemcontroller 120. Then, the system controller 120 evaluates an integratedvalue for each bayer pixel in each region to thereby performlight-metering processing.

The image data controller 115 is connected with a timing-pulsegeneration circuit 114 that outputs a drive pulse signal to the imagingelement 112. An analog/digital (A/D) converter 113 receives a timingpulse generated by the timing-pulse generation circuit 114 and thenconverts an analog signal corresponding to the object image output fromthe imaging element 112 into a digital signal and then sends it to theimage data controller 115. The obtained digital image data istemporarily stored in a dynamic random access memory (DRAM) 121. TheDRAM 121 is used for temporarily storing image data before beingsubjected to processing or data conversion into a predetermined format.An image compression circuit 119 is connected with a storage sectionincluding a storage medium 401.

The image compression circuit 119 performs compression or conversion ofimage data (e.g., JPEG) stored in the DRAM 121 to store the convertedimage data in the storage medium 401. Examples of the storage medium 401include a magnetic storage medium, a semiconductor memory, and the like.

A digital/analog (D/A) converter 116 that converts digital image datainto an analog signal is connected with an image display circuit 118 viaan encoder circuit 117. The image display circuit 118 is constituted bya color liquid crystal display element or the like for displaying theimage data captured by the imaging element 112. The image datacontroller 115 converts the image data on the DRAM 121 into an analogsignal by the D/A converter 116 and outputs the analog signal to theencoder circuit 117. The encoder circuit 117 converts the output of theD/A converter 116 into a video signal (e.g., National Television SystemCommittee (NTSC) signal) necessary for driving the image display circuit118.

Furthermore, the system controller 120 and the image data controller 115have a focusing point detection function. The image data controller 115causes the corrected image data to pass through a filter having apredetermined frequency characteristic and evaluates a contrast in apredetermined direction of the image signal obtained after gammaprocessing. When the system controller 120 receives the result, thesystem controller 120 communicates with the optical system controlsection 207 to adjust a focal position so that the contrast evaluationvalue is higher than a predetermined level.

Further, the system controller 120 is connected with an operationdisplay circuit 123 that causes an external liquid crystal displaydevice 124 and an internal liquid crystal display device 125 to displaythe operation mode information of the digital camera and exposureinformation such as a shutter second time, diaphragm value, or the like.A communication interface circuit 126 communicates with externalappliances. The operating means for providing user's or photographer'soperation instructions to the system controller 120 are, for example, asfollows.

The system controller 120 is further connected with a main electronicdial 131, a determination SW 132, and a photographic mode selectionbutton 130 that sets a mode so that an electronic camera can perform anoperation desired by a user.

-   An imaging-mode selection button (130): A button for setting a mode    to cause a camera to execute an operation desired by a user.-   A main electronic dial (131) and a determination SW (132).-   A focusing point-state detection point selection button (133): A    button for selecting a focus detection position to be used from    among a plurality of focus detection positions sensed by the AF    sensor unit 104.-   An auto focus (AF) mode selection button (134) and a light-metering    mode selection button (135).-   A release SW1 (136): A switch for starting an imaging preparation    operation such as light-metering, focus detection, and the like.-   A release SW2 (137): A switch for starting an imaging operation.-   A viewfinder mode selection SW 138: A switch for switching between    an optical viewfinder mode that can confirm light flux passing    through the eyepiece lens 106 and a live view display mode that can    sequentially display the image received by the imaging element 112    on a display section with the image display circuit 118.

In addition, a power supply control circuit, a power supply sectioncontrolled by the circuit, and the like are provided in the camera body100. However, the illustration and the explanation will not be givenhere because they are already known.

A strobe device 300 is detachably mounted on the camera body 100 via amounting mechanism (not shown). A mounting part includes a group ofelectric contact points 310. The group of contact points 310 transmitand receive a control signal, a state signal, a data signal, and thelike between the camera body 100 and the strobe device 300. The strobedevice 300 is provided with a control terminal for light emitting timingand a communication terminal with the system controller 120. The strobedevice 300 includes a xenon (Xe) tube 301 and a reflection shade 302. Alight emission control circuit 303 is constituted by an insulated gatebipolar transistor (IGBT) or the like that controls the emission oflight from the Xe tube 301. A charging circuit 304 generates an voltageof about 300 V to power the Xe tube 301. A battery that powers thecharging circuit 304 or the like is used for a power source 305. Astrobe control section 306 controls light emission from a strobe,charging, and the like, as well as communicates with the systemcontroller 120 of the camera side.

Next, the diaphragm control circuit 206 will be described in detail withreference to FIG. 2.

The diaphragm control circuit 206 performs the drive control of astepping motor used for the diaphragm driving mechanism 205. A BLK-A isa drive section for one phase of a stepping motor, and a BLK-B is also adrive section having the same circuit configuration as that of the drivesection BLK-A. A decoder 216 generates a drive control signal to thedrive sections BLK-A and BLK-B in response to a control signal from theoptical system control section 207. The output signal of the decoder 216is sent to a NOT (logical NOT) circuit 221, a NOR (negative OR) circuit223, and an NPN transistor 227 to be described below. The output signalof a NOT circuit 211 is sent to a NOR circuit 222 and an NPN transistor226 to be described below.

Upper PNP transistors 224 and 225 and lower NPN transistors 226 and 227constitute a so-called “H-bridge”. The output signal of the NOR circuit222 is input to the base of the PNP transistor 224, and the outputsignal of the NOR circuit 223 is input to the base of the PNP transistor225. The respective emitters of the PNP transistors 224 and 225 areconnected to each other to be supplied with a predetermined power supplyvoltage. Also, the respective emitters of the NPN transistors 226 and227 are connected to each other and are grounded via a resistor 229. Theresistor 229 is used for the detection of drive current for a steppingmotor.

The plus input terminal of a comparator 228 is connected with theungrounded terminal of the resistor 229, and the minus input terminalthereof is connected with a reference voltage generation circuit 220.The output signal of the comparator 228 is input to the NOR circuits 222and 223. The reference voltage generation circuit 220 generates areference voltage shown in FIG. 4 in response to the outputs VA0 to VA3(or VB0 to VB3) of the decoder 216. Note that in FIG. 4, the voltageVOUT in response to the logical values of VA0 to VA3 is represented as apercentage with respect to a predetermined voltage in a tabular format.

Next, the configuration of a stepping motor will now be described indetail. In an A-phase stator 251, an A-phase coil 252 is wound around anA-phase yoke. One end of the A-phase coil 252 is connected with thecollector of the PNP transistor 224 and the NPN transistor 226, and theother end is connected with the collector of the PNP transistor 225 andthe NPN transistor 227.

In a B-phase stator 253, a B-phase coil 254 is wound around a B-phaseyoke. Although it is not shown, the B-phase coil 254 is also connectedwith the collector of each of transistors of BLK-B in the same manner asdescribed above.

A rotor 250 is disposed opposite to the protrusion of each stator and ismagnetized alternately between the N-pole and the S-pole (in FIG. 2,only two poles are shown).

Next, the terminals of the diaphragm control circuit 206 will now bedescribed. As used herein, each of the codes shown in FIG. 2 has ameaning as follows:

CK: A clock input terminal to which the driving frequency signal of thestepping motor is input.

DIR: An input terminal for specifying the rotational direction of thestepping motor.

EN: An input terminal for specifying the drive and stop of the steppingmotor.

M0, M1: An input terminal for specifying the drive mode of the steppingmotor.

H/L: An input terminal for specifying the drive current of the steppingmotor.

The input signals to the aforementioned terminals are supplied by theoptical system control section 207 shown in FIG. 1. As used herein, eachof the other codes shown in FIG. 2 has a meaning as follows:

SG: A signal ground terminal.

VDD: A power supply input terminal for control circuit.

VM: A power supply input terminal for motor drive. MA,/MA: An A-phaseconnection terminal of the stepping motor.

MB,/MB: A B-phase connection terminal of the stepping motor.

PG: A ground terminal of the motor system.

FIG. 3 shows the drive modes of the stepping motor with respect tosignals input to the input terminals M0 and M1. When M0=0 and M1=0, theSLEEP mode is set, and the diaphragm control circuit 206 places itselfin a low-power mode. When M0=0 and M1=1, 1-phase excitation drive isset. When M0=1 and M1=0, the 1-2-phase excitation drive is set. WhenM0=1 and M1=1, the micro-step drive is set. The aforementioned drivingmethods will be described in detail below.

FIG. 4 is a diagram for explaining the output voltage of the referencevoltage generation circuit 220 provided in the diaphragm control circuit206. Each of VA3 to VA0 is the output signal of the decoder 216. Inresponse to these output signals VA3 to VA0, the reference voltagegeneration circuit 220 generates a VOUT (reference voltage). Thereference voltage is input to the minus input terminal of the comparator228. The comparator 228 compares the voltage generated in response tothe current of the stepping motor flowing through the resistor 229 withthe reference voltage. When the detection voltage detected by theresistor 229 is higher than a reference value, i.e., when the currentflowing through the stepping motor is high, the output of the comparator228 is set to a HI (high) level. The transistors 224 and 225 are turnedoff by the output signal of the NOR circuits 222 and 223, the currentflowing through the A-phase coil 252 of the stepping motor is shut off.Consequently, the current flowing through the resistor 229 is shut offto zero, and thus the output signal of the comparator 228 becomes a LO(low) level. The transistors 224 and 225 are turned on by the outputsignal of the NOR circuits 222 and 223. By repeating the on-off cycle,the current flowing through the A-phase coil 252 of the stepping motoris controlled so as to be made substantially constant in response to theoutput voltage of the reference voltage generation circuit 220. Also,the VA3 signal shown in FIG. 4 is connected with the H/L input terminalof the decoder 216. With this arrangement, the drive current of thestepping motor can be switched between full-range (0 to 100% inclusive)and half-range(0 to 50% inclusive) in response to the input signal ofthe H/L terminal. In the drive section BLK-B show in FIG. 2, theoperation for generating a reference voltage shown in FIG. 4 issimilarly carried out by the output signals VB0 to VB3.

Next, the drive control of the stepping motor will now be described withreference to FIGS. 5 and 6.

In FIG. 5, CLK represents a reference clock signal to be input to theclock input terminal CK shown in FIG. 2. Also, φA and φB representcurrent waveforms flowing through the coils 252 and 254 of the steppingmotor, respectively, and a plurality of transverse lines represents acurrent level corresponding to the voltage of the reference voltagegeneration circuit 220 shown in FIG. 4.

In the description of the drive methods of the stepping motor, the drivemethods are generally classified into three types: 1-phase excitation,2-phase excitation, and 1-2-phase excitation as shown in FIGS. 5A, 5B,and 5C. FIG. 5D shows the current control of a micro-step drive in whichone step of the motor is controlled more finely by controlling thecurrents flowing through the coils 252 and 254 in a stepwise manner inthe 1-2-phase excitation drive.

The waveforms shown in FIG. 5 shows the phase relationship of thecurrents flowing through the coils 252 and 254. In practice, sincenoise, vibration, and the like are removed, the actual waveform is not asimple rectangular waveform similar to the excitation waveform shown inFIG. 5, but may be a trapezoidal waveform with its rising edge andfalling edge being inclined or another waveform rising and falling in asinusoidal fashion. However, the phase relationship basically remainsthe same.

FIG. 6 shows how the rotor 250 of the stepping motor is rotated by therespective drive methods. The stepping motor used herein has two coils,i.e., an A-A bar phase and a B-B bar phase, the magnetic field generatedby each coil is directed in a position shown in FIG. 6. Note that inFIG. 6, the A bar phase and the B bar phase are represented by adding“-” over the symbols A and B, respectively.

In the case of 1-phase excitation drive, the magnetic poles of the rotorare always moved to face the magnetic poles of the stator as shown inFIG. 6A. To be more specific, with a starting point from the magneticpole of the rotor denoted by a black point shown in FIG. 6, the S-poleof the rotor shown by a black point is disposed facing to the A-phase inthe state shown in (1) of FIG. 6A. When it reaches to the state shown in(2) of FIG. 6A, the B-phase becomes the N-pole, and the magnetic pole ofthe rotor denoted by a black point is moved to a position facing to theB-phase. Specifically, in the present example, the rotation of 22.5degrees can be obtained with a change in the magnetic pole of thestator.

In 1-phase excitation drive, the rotation stepping angle per step islarge, which is not suitable when highly accurate positioning isrequired, such as for a diaphragm. However, the magnetic poles of therotor are always disposed to face the magnetic poles of the stator,which is convenient for holding the diaphragm position. This is becausethe rotation of the rotor can be prevented since the permanent magnetrotor is attracted to the metal stator to some extent even if a holdingcurrent does not flow while the motor is stopped.

In the case of the 2-phase excitation drive, the current shown in FIG.5B is supplied to the coil of each phase, and thus the rotation of therotor 250 is as shown in FIG. 6B. The rotor is moved so as to face apoint intermediate between the adjacent magnetic poles of the stator. Asin the 1-phase excitation drive, the rotor is rotated at an angle of22.5 degree with a change in the magnetic pole of the stator. However,two excitation phases of the coils are always provided with electricity,whereby more powerful drive torque and drive speed can be obtained.

In the case of the 2-phase excitation drive, the magnetic pole of therotor is always stopped so as to face a position intermediate betweenthe adjacent magnetic poles of the stator. When the current supplied tothe stator coil is shut off in a stop state, it would be expected thatthe rotor is moved at an angle of ½ step so as to face the stator ofeither left or right. The angle displacement of ½ step is caused by anerror in the assembly of the magnetic poles of the stator, a slightdeviation of the stop position, and the like. To be more specific, inthe case of the 2-phase excitation drive, the coil needs to be excitedeven in the stop state, and thus the coil must be provided withelectricity during a stop state in order to supply a minimum currentsufficient to assure the magnetic field for holding the stop position ofthe rotor. The magnitude of holding current ranges from tens to hundredsmA per coil, and considerable heat is produced.

As shown in FIG. 5C, in the case of the 1-2-phase excitation drive, the1-phase excitation and the 2-phase excitation are alternately repeated.Hence, as shown in (1) of FIG. 6C, when the rotation of the rotor isstarted from the position facing the magnetic pole of the stator, therotor is moved in a position between the magnetic poles of the stator asshown in (2) of FIG. 6C in the next change in the magnetic pole. In thefurther change in the magnetic pole, the rotor is moved to face the nextmagnetic pole as shown in (3) of FIG. 6C. Hence, the rotation amountobtained by a change in the magnetic pole of the stator is 11.25degrees, which corresponds to ½ that of the 1-phase excitation and the2-phase excitation, and the rotational position of the rotor can therebybe controlled more finely. However, for the same reason as describedabove, when the stop position of the rotor is held, a current may beshut-off in the case where the stop position is the 1-phase excitationphase, whereas a holding current is necessary in the case where the stopposition is the 2-phase excitation phase.

In the case of the micro-step drive, the currents flowing through theA-phase coil 252 and the B-phase coil 254 are controlled with respect tothe 1-2-phase excitation shown in FIG. 5C in a stepwise manner as shownin FIG. 5D, whereby the rotational position of the rotor can becontrolled more finely. In this manner, the diaphragm can be drivensmoothly and quietly with high accuracy, which is suitable for diaphragmcontrol or the like during video photography. In the micro-step drive,when the stop position of the rotor is the 1-phase excitation phase, thestop position can be held even if a current is shut off, as in the1-2-phase excitation drive. However, in other stop positions includingthe 2-phase excitation phase, a current needs to flow so as to hold thestate, and thus a current cannot be shut off.

While the detailed description has been made of the diaphragm controlcircuit 206, the diaphragm driving mechanism 205, and the drive for thestepping motor, the lens control circuit 204 can be controlled in thesame manner since it is similarly structured in the same manner.

Next, an exemplary operation according to an embodiment of the presentinvention will now be described in detail with reference to FIGS. 7 to10. Note that the present operation is carried out in accordance with aprogram that is interpreted and executed by the system controller 120.

FIG. 7 is a flowchart illustrating a video photography operation. Whenthe power supply of a camera is switched on in step S101, a program isloaded into the system controller 120, and an initialization processrequired for a camera operation and an initialization of controlparameters are carried out. At this time, when the imaging lens unit 200is mounted on the camera body 100, the optical system control section207 notifies the system controller 120 of the camera body 100 about thelens ID (identification information) and information on various types oflenses. Lens information includes flag information for determiningwhether or not the diaphragm driving corresponds to a micro-step drive.Flag information is used for the switching of the operation modes to bedescribed below.

After completion of the initialization process, the start of videorecording is determined in step S102. When to the camera through auser's operation instructions is notified about a start command forvideo recording, the quick return mirror 102 and the sub-mirror 103 areretracted from the optical path, and thus the light flux passing throughthe imaging lens 201 reaches the imaging element 112. Then, the processadvances to step S103. During any period in which the start of videorecording is not determined, the process in step S102 is repeated.

A video recording operation is started in step S103. The image signalthat has been continuously acquired and read out from the imagingelement 112 is sequentially displayed on a display section such as aliquid crystal display or the like disposed in the back of the cameracasing, and the image data is then stored in the storage medium 401.

Here, a description will be given of an operation during videorecording. After the exposure operation of the imaging element 112 hasbeen performed, the accumulated charge of each pixel within the imagingelement 112 is read out as an image signal. This reading out operationis performed in synchronization with a control pulse verticalsynchronization signal VD and a horizontal synchronization signal HD.The VD signal is a signal that represents one frame of imaging. Forexample, a command is received from the system controller 120 per 1/30second, and the timing-pulse generation circuit 114 transmits a signalto the imaging element 112. Also, the HD signal is the horizontalsynchronization signal of the imaging element 112. A number of pulsescorresponding to the number of horizontal lines are transmitted in theperiod of one frame so as to control the horizontal lines. Also, a pixelreset is carried out per horizontal line such that a horizontal line issynchronized with an HD signal to become the set charge accumulationtime. When an accumulation/read-out operation is executed by a VD signalor an HD signal, an accumulation operation for the next frame is startedin accordance with the transmission timing of a VD signal. Also, thereadout image signal is transferred to the image data controller 115 tobe subjected to processing such as defective pixel correction or thelike. Through image processing, the processed image signal istransmitted to the image display circuit 118 of the display sectiondisposed on the rear face or the like of the camera. With regard tovideo recording, likewise, the readout signal is subjected to imageprocessing and then transmitted to the storage medium 401 so as tosequentially record image data. It should be noted that these arewell-known techniques and no further description will be made thereon.

In step S104, a light-metering operation is performed based on thesignal that has been read out in step S103. During light-metering, theimage data controller 115 divides the image signal into regions andsupplies an value integrated for each bayer pixel in each region to thesystem controller 120. The system controller 120 evaluates the obtainedintegration signal to thereby perform light-metering processing. In thenext step S105, an automatic exposure control for a moving image isperformed based on the light-metering value determined in step S104 andlens information.

An exemplary automatic exposure control performed during video recordingwill now be described with reference to FIG. 8. In the presentembodiment, the operation modes for driving the stepping motor fordriving the diaphragm are switched depending on the type of the lenssection.

First, in step S201, the control value By of the camera is calculatedbased on the light-metering value (Bv value) determined in step S104such that moving image exposure becomes a predetermined brightnesslevel. Here, the By value is an index value that indicates brightnessrepresented by the following formula [Formula 1], and is a value that isuniquely determined by the combination of Tv, Av, and Sv in which theobject with brightness represented by the index value becomes anappropriate exposure level. Tv represents a shutter-speed, Av representsa diaphragm value, and Sv represents a sensitivity. Also, the controlvalue By is a set value determined by the combination of Tv, Av, and Svfor controlling the camera.

By=Tv+Av−Sv   [Formula 1]

In general, for moving image exposure, a variation of exposure betweensuccessive frames such as flickering, hunting, and the like occurs.Hence, it is desirable that exposure be performed in slow and smoothresponse to some degree of time constant. FIG. 9 shows an exemplarygraph illustrating the variation with time of the control value By andthe variation with time of the By value. When the brightness of theobject changes during video recording, for example, the response of thelight-metering value (Bv value) and the control value (control value Bv)as shown in FIG. 9 is expected. In the present example, when thebrightness value of the object rises from Bv3 and changes to Bv6 at thetime t0, the control value By of the camera slowly and smoothly changesfrom time t0 to the time t1 to reach an appropriate exposure level (Bv6)at the time t1. Also, when the brightness value of the object drops fromBv6 and changes to Bv4 at the time t2, the control value By of thecamera slowly and smoothly changes from the time immediately before t2to the time t3 in the same manner to reach an appropriate exposure level(Bv4) at the time t3. Such a change is realized by determining thecontrol value By using a predetermined response function.

In step S202 shown in FIG. 8, it is determined whether or not theimaging lens unit 200 mounted on the camera body 100 corresponds to thediaphragm driving in the micro-step drive (hereinafter referred to as“micro-step diaphragm driving”) based on the lens information acquiredin step S101 shown in FIG. 7. In the case of the imaging lens unit notcorresponding to the micro-step diaphragm driving, the process advancesto step 203, whereas in the case of the imaging lens unit correspondingto the micro-step diaphragm driving, the process advances to step S210.

The operation mode for exposure control is respectively selecteddepending on whether or not the imaging lens unit 200 mounted on thecamera body 100 corresponds to the micro-step diaphragm driving.Specifically, when the imaging lens unit 200 mounted on the camera body100 includes a diaphragm driving mechanism corresponding to themicro-step driving, the operation mode is switched to a first operationmode (silent drive mode). In the first operation mode, a phase by whichthe position of a rotor is not held when a current to be supplied to aplurality of coils is shut off is used. Also, when the imaging lens unit200 mounted on the camera body 100 does not include a diaphragm drivingmechanism corresponding to the micro-step drive, the operation mode isswitched to a second operation mode. In the second operation mode, onlya phase by which the position of a rotor is held when a current suppliedto the plurality of coils is shut off is used. By switching between twooperation modes, the optimum program profile is respectively selected inresponse to the drive characteristics of the diaphragm. In other words,in the first operation mode, a first program profile in which the chargeaccumulation time is fixed so as to change the diaphragm value forperforming exposure control in a predetermined brightness range isselected (see FIG. 10B). In the second operation mode, a second programprofile in which the diaphragm value is fixed so as to change the chargeaccumulation time for performing exposure control in a predeterminedbrightness range is selected (see FIG. 10A). With this arrangement,exposure flickering or the like, which is caused by the diaphragmoperation, can be suppressed by reducing the number of diaphragm drivingtimes even for an imaging lens unit which provides poor controllabilityof the diaphragm. In addition, an optimum current control for thediaphragm can be switched in response to the operation modes.

In step S203, an exposure control value is determined in accordance withthe second program profile. FIG. 10A shows an exemplary program profilefor an imaging lens unit that does not correspond to the micro-stepdiaphragm driving. With this program profile, the charge accumulationtime, the diaphragm value, and the gain value for controlling the cameraare determined. As features of the second program profile, the set value(Av) of the diaphragm is discretely selected (see F2.8, F8, and F22 inFIG. 10A), and a fine exposure value is set by changing the chargeaccumulation time (Tv) and the gain (Sv). Here, the actually selecteddiaphragm set value is intended to be a rounded value such that it isthe stop position of the 1-phase excitation drive for the stepping motorfor driving the diaphragm. Also, the diaphragm set value has ahysteresis (indicated by the dashed lines) of ±3 steps. Hence, once thediaphragm driving occurs, the diaphragm driving does not occur again forchanges in brightness within the range of the hysteresis. Specifically,exposure flickering or the like can be suppressed by reducing the numberof diaphragm driving times even for an imaging lens unit which providespoor controllability of the diaphragm. In contrast, when the diaphragmdriving occurs, flickering, rapid depth change, and diaphragm drivingnoise are recorded in a moving image.

In step S204, the system controller 120 compares the current diaphragmvalue controlled by the camera with the diaphragm set value determinedin step S203 so as to determine whether or not the diaphragm drivingoccurs. A determination of the presence or absence of the diaphragmdriving is carried out by comparing the amount of change in position ofthe diaphragm, i.e., the difference between the current diaphragm valueand the set value with a predetermined amount (a preset referencevalue). When the amount of change in the position of the diaphragm isequal to or greater than a predetermined amount as a result of thedetermination and thus the diaphragm driving occurs, the processadvances to step S205, whereas when the amount of change in the positionof the diaphragm is less than a predetermined amount as a result of thedetermination and thus the diaphragm driving does not occur, the processadvances to step S206.

In step S205, a request for driving the diaphragm is made from thesystem controller 120 to the optical system control section 207 suchthat the diaphragm value becomes the one determined in step S203. Atthis time, the 1-phase excitation drive described above is selected asthe drive method of the stepping motor, and thus the rotor of thestepping motor is always stopped at a position facing the magnetic polesof the stator.

In step S206, since the diaphragm driving does not occur, a shut-offcommand for diaphragm holding current is transmitted from the systemcontroller 120 to the optical system control section 207, then a currentsupplied to the stepping motor is shut off and a holding current therebybecomes zero. At this time, the stop position of the diaphragm iscontrolled by the drive control in step S205 such that the steppingmotor is always stopped at a position where the rotor and the magneticpoles of the stator face to each other. Hence, since the permanentmagnet rotor attracts the metal stator to some extent even if a currentis shut off, a shift of the stop position of the diaphragm can beprevented.

After performing steps S205 and S206, the process advances to step S207.Here, in order to obtain the charge accumulation time and the gaindetermined in step S203, the timing-pulse generation circuit 114, theA/D converter 113, and the image data controller 115 are set to therebyperform exposure control of a moving image, and a series of processingsteps described above is ended.

In contrast, when the imaging lens unit 200 corresponding to themicro-step diaphragm driving is mounted on the camera body 100 in stepS202, the process proceeds to step S210.

In step S210, the exposure control value is determined in accordancewith the first program profile.

FIG. 10B shows an exemplary program profile for an imaging lens unitthat corresponds to the micro-step driving. With this program profile,the charge accumulation time, the diaphragm value, and the gain valuefor controlling the camera are determined. As features of the firstprogram profile, settings of the charge accumulation time and the gainare fixed in a predetermined brightness range (see from Ev10 to Ev16 inFIG. 10B) so as to change the diaphragm value for performing exposurecontrol. At this time, since the diaphragm is driven by the micro-stepdrive, flickering and rapid depth change upon exposure of a moving imageto be recorded, and diaphragm driving noise during recording aresuppressed. In contrast, in the micro-step drive described above, themagnetic pole of the rotor may be stopped so as to face at a positionintermediate between the adjacent magnetic poles of the stator. Hence,when the electricity supplied to the stepping motor is shut off underthe stop state, the rotor may be moved to a stable position.

In step S211, the system controller 120 compares the current diaphragmvalue controlled by the camera with the diaphragm set value determinedin step S210 so as to determine whether or not the diaphragm drivingoccurs. At this time, when the difference between the current positionof the diaphragm and the diaphragm set value determined by calculationlies within a predetermined range, the system controller 120 candetermines that the diaphragm driving is not necessary. This is becauseof the suppression of a hunting phenomenon to be described below, inwhich a shut-off state of current supplied to the stepping motor and areactivation state frequently happen, occurs, which results in irregulardiaphragm driving. Also, the exposure error within a predetermined rangemay be corrected by setting the charge accumulation time or the gain soas to obtain an appropriate exposure level. When the diaphragm drivingoccurs in step S211, the process advances to step S212, whereas when thediaphragm driving does not occur, the process advances to step S213.

In step S212, a request for driving the diaphragm is made from thesystem controller 120 to the optical system control section 207 suchthat the diaphragm value becomes the one determined in step S210. Atthis time, the micro-step driving is selected as the driving method ofthe stepping motor. Then, the process advances to step S207.

In step S213, the system controller 120 obtains status information fromthe imaging lens unit 200 so as to determine whether or not the steppingmotor is provided with electricity. When a current supplied to thestepping motor is shut off, the process advances to step S207, whereaswhen the stepping motor is provided with electricity, the processadvances to step S214.

In step S214, the system controller 120 obtains status information fromthe imaging lens unit 200 so as to determine the stop phase of the rotorof the stepping motor. When the system controller 120 determines thatthe stop phase of the rotor is a stable phase, i.e., the rotor and themagnetic poles of the stator face to each other, the process advances tostep S215. Here, a control command is transmitted to the imaging lensunit 200 to shut off the current supplied to the stepping motor, wherebya holding current becomes zero. On the other hand, when the systemcontroller 120 determines that the stop phase of the rotor is anunstable phase, i.e., the rotor is positioned intermediate between thestator and the stator, the process proceeds to step S216.

In step S216, a request for micro-step driving is made from the systemcontroller 120 to the optical system control section 207 such that thestop phase shifts from the unstable phase to the stable phase with thesmallest rotation angle of the rotor. Exposure of a moving image isstabilized by this process. When the diaphragm driving does not occur,the position of the rotor of the stepping motor is guided to the stablephase. Then, the process advances to steps S215 and S207.

The explanation will be continued with reference again to FIG. 7. Whenthe end of video recording is reported by an operation button (notshown) in step S106, the system controller 120 ends videorecording/photographing to return the quick return mirror 102 and thesub-mirror 103 from their retracted position. Then, processing is ended.When the end of video recording/photographing is not reported, theprocess returns to step S103 so as to continue videorecording/photographing.

While in the present embodiment, the phase in which the rotor does notrotate even if the electricity supplied to the stepping motor is shutoff in the case of the 1-phase excitation drive is employed, the presentinvention is not limited thereto. The phase in which the rotor does notrotate even if the electricity supplied to the stepping motor is shutoff in the case of the 2-phase excitation drive may also be employed.

As described above, in the present embodiment, a method for controllinga stepping motor and a method for controlling energization of thestepping motor are switched in response to the imaging lens unit mountedon the camera body. With this arrangement, an exposure control for afavorable moving image can be realized and an optimum current controlcan be made in accordance with the operation modes, whereby the lowerpower consumption of the imaging apparatus can be realized.

While the embodiments of the present invention have been described withreference to exemplary embodiments, it is to be understood that theinvention is not limited to the disclosed exemplary embodiments. Thescope of the following claims is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalentstructures and functions.

This application claims the benefit of Japanese Patent Application No.2009-224547 filed Sep. 29, 2009 which is hereby incorporated byreference herein in its entirety.

1. An imaging apparatus comprising: a lens section having a lens group and a diaphragm; a stepping motor that drives the diaphragm by means of the excitation of a plurality of coils; a control unit configured to control the stepping motor; an exposure control unit configured to control exposure during video photography by changing a diaphragm value, a charge accumulation time or a gain of an imaging element; and a determination unit configured to determine whether or not the amount of change in the position of the diaphragm is equal to or greater than a predetermined amount, when a current supplied to the plurality of coils is shut off; wherein the control unit switches operation modes in association with the driving of the stepping motor depending on the type of the lens section; in a first operation mode using a phase by which the position of a rotor is not held when a current supplied to the plurality of coils is shut off, the control unit controls the stop position of the diaphragm based on the output of the exposure control unit and the determination result obtained by the determination unit; and in a second operation mode using only a phase by which the position of a rotor is held when a current supplied to the plurality of coils is shut off, the control unit performs diaphragm control based on the output of the exposure control unit.
 2. The imaging apparatus according to claim 1, wherein, when the determination unit determines that the amount of change in position of the diaphragm is less than a predetermined amount, the exposure control unit performs exposure compensation while changing the charge accumulation time or the gain without changing the position of the diaphragm.
 3. The imaging apparatus according to claim 1, wherein, when the determination unit determines that the amount of change in position of the diaphragm is equal to or greater than a predetermined amount, the control unit changes the position of the diaphragm.
 4. The imaging apparatus according to claim 1, wherein the exposure control unit, in the first operation mode, selects a first program profile including exposure control through which the charge accumulation time is fixed so as to change the diaphragm value, and, in the second operation mode, selects a second program profile including exposure control through which the diaphragm value is fixed so as to change the charge accumulation time.
 5. The imaging apparatus according to claim 4, wherein the control unit performs micro-step driving for the stepping motor in the first operation mode, and performs 1-phase excitation driving for the stepping motor in the second operation mode.
 6. The imaging apparatus according to claim 5, wherein, when it is determined that the lens section has a diaphragm driving mechanism corresponding to the micro-step driving, the exposure control unit selects the first program profile in the first operation mode, whereas when it is determined that the lens section does not have a diaphragm driving mechanism corresponding to the micro-step driving, the exposure control unit selects the second program profile in the second operation mode.
 7. The imaging apparatus according to claim 6, wherein, when the control unit controls in the first operation mode that the amount of change in position of the diaphragm is less than a predetermined amount, the control unit controls the stop position of the stepping motor and then cuts off the current supplied to the plurality of coils, whereas when the determination unit determines in the second operation mode that the amount of change in position of the diaphragm is less than a predetermined amount, the control unit cuts off the current supplied to the plurality of coils.
 8. A method for controlling an imaging apparatus comprising a lens section having a lens group and a diaphragm; a stepping motor that drives the diaphragm by means of the excitation of a plurality of coils; and a control unit configured to control the stepping motor, the method comprising: an exposure control step of controlling exposure during video photography by changing a diaphragm value, a charge accumulation time, or a gain of an imaging element; a determination step of determining whether or not the amount of change in position of the diaphragm is equal to or greater than a predetermined amount, when a current supplied to the plurality of coils is shut off; and a step of switching operation modes in association with the driving of the stepping motor depending on the type of the lens section, in a first operation mode using a phase by which the position of a rotor is not held when a current supplied to the plurality of coils is shut off, controlling the stop position of the diaphragm based on a set value set in the exposure control step and the determination result obtained by the determination step, and in a second operation mode using only a phase by which the position of a rotor is held when a current supplied to the plurality of coils is shut off, performing diaphragm control based on the set value set in the exposure control step. 